OTOH if someone sensible from a crew doesn’t speak to them its possible that idiots will take matters into their own hands and throw bottles/cans – at this point the cops can declare whats called a “code 00” incident (cops being attacked) and use whatever force they feel is necessary to close down the event

Exactly. Always a good idea to approach them as soon as they turn up and it can also be wise to make sure you talk to them always from the bulk of people for the very reason GL states. Most people at parties are safe as houses but there will always be a minority that could mess it up for others.